The present invention relates to lasers and, more particularly, to a laser having an etalon incorporated therein in a manner providing maximum mechanical and thermal stability of the etalon position relative to the laser optical axis. The invention further relates to a laser having a temperature controlled etalon and a heater for the etalon so positioned that efficient and sensitive etalon temperature stabilization is achieved without contamination of the laser optical cavity by the etalon heater components which might, for example, out-gas at operation temperatures.
Typically, the beam emanating from an ion laser includes at each of its normal frequency outputs, a longitudinal mode spectrum made up of a plurality of spectrum lines adjacent the output frequency. In certain uses to which a laser is put, however, such as in holography, high resolution spectroscopy and interferometry, it is desirable and sometimes necessary that the output of the laser be restricted to a single frequency mode. To this end, etalon assemblies have been incorporated into ion lasers to act, in effect, as band pass transmission filters. The etalon, when properly oriented, will pass frequencies close to its transmission peak and reject by reflection frequencies outside the etalon pass band.
In general, there are two different types of etalons used to limit the output of ion lasers to a single longitudinal mode. There is the solid etalon in which the end faces of a solid, transparent body are used as the etalon reflecting surfaces, and there is the air-spaced etalon which is basically two spaced reflecting surfaces defining a resonant cavity. A solid etalon is inherently unstable under varying temperature conditions. That is, changes in the index of refraction of the material from which the etalon is made due to temperature changes will result in so-called "mode hopping" and frequency drift. The air-spaced etalon was developed to overcome this difficulty. Although the air-spaced etalon produces good thermal stability, it must be tuned to different desired modes by changing its angular orientation within the optical cavity. The result is a "walk-off" power loss. Moreover, such an air-spaced etalon will have two more optical surfaces than a solid etalon through which radiation must pass, often resulting in reduced laser power.
In view of the disadvantages inherent in air-spaced etalons, those in the art have overcome the temperature instability of solid etalons by including a heating element with the same to stabilize its temperature. The inclusion of the heating element also enables the solid etalon to be tuned by taking advantage of the change in the index of refraction with temperature. That is, by intentionally changing the temperature of the material of the etalon, the longitudinal mode selected by the etalon for transmission can be correspondingly changed.
Presently available laser etalon assemblies, however, are not ideal and their employment in a laser in-of-itself does not always provide the accurate single frequency operation required for some uses. In this connection, it is quite important that the mechanical positioning of the etalon with respect to the remainder of the optical cavity be maintained quite stable in order to prevent mode hopping and frequency drift. Moreover, present constructions utilizing a heater for a solid etalon are not satisfactory in that gases and the like which evolve from the surfaces of the heater elements and other components contaminate not only the etalon reflecting surfaces, but also other parts of the optical cavity.